Brown algae such as Sargasso (scientific name: Sargassum fulvellum) can grow in large amounts and do not compete with food resources, so that they are expected to serve as raw materials for production of biofuel. However, a major component of brown algae is alginate (acidic polysaccharide), accounting for about 30% to 60% of the dry alga body. Studies that have been conducted to date focus mainly on the use of cellulose in brown algae. A method for converting alginate (polysaccharide comprising uronic acid (saccharide with a carboxylated terminus (at position 6) of the main chain), which is a major component of brown algae, to biofuel ethanol, has not yet been established.
Technology for producing ethanol from alginate has been examined. It has been reported that a pit-forming bacterium of a Sphingomonas sp. strain A1 (hereinafter, referred to as the “strain A1”), produces a trace amount of ethanol from alginate (see non-patent document 1). However, the ethanol productivity of the strain A1 is extremely low, so that production thereof on a commercial basis and/or commercialization thereof is extremely difficult.
The metabolic pathway for production of ethanol from alginate has been revealed to some extent. It has been inferred that the strain A1 degrades alginate to a monosaccharide (α-keto acid) with endo- and exo-lyases, converts it to 2-keto-3-deoxy-D-gluconate (KDG) with α-keto acid reductase, and converts the resultant to pyruvic acid by a reaction with enzymes, A1-K (kinase) and A1-A (aldolase) (see non-patent document 2). Pyruvic acid mainly enters the TCA cycle and is then used for ATP (energy source) production under aerobic conditions, and it produces ethanol in a microaerophilic environment.
The pathway from uronic acid (monosaccharide) to pyruvic acid in bacteria of the genus Pseudomonas has been reported, and the presence of α-keto acid reductase has also been reported (see non-patent documents 3 and 4). Also, an enzyme group (KDG-kinase, KDG-aldolase) involved in metabolism of 2-keto-3-deoxy-D-gluconate (KDG) and genes thereof in Escherichia coli have been revealed (http://www.genome.jp/dbget-bin/get_pathway?org_name=eco&mapno=00040).
Meanwhile, a gram-negative bacterium Zymomonas mobilis (lacking alginate assimilation capacity) is known as a bacterium having strong ethanol production capacity. It has been reported that Zymomonas mobilis produces ethanol from glucose, fructose, or sucrose with efficiency as high as 2.5 times greater than that of yeast. Furthermore, ethanol production technology that involves incorporating Zymomonas mobilis-derived pyruvate decarboxylase and alcohol dehydrogenase into Escherichia coli and thus causing ethanol production from glucose has been reported (see patent document 1). This strain is currently industrially used as the strain KO11 (ATCC55124) for bioethanol production from cellulose-based biomass. Also, incorporation of Zymomonas mobilis-derived pyruvate decarboxylase and alcohol dehydrogenase into Gram-negative bacteria (e.g., Erwinia, Klebsiella, and Xanthomonas) other than Escherichia coli has also been reported (see patent document 2).